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Lichens' secret symbiotic threesome

American Association for the Advancement of Science

The classical view of lichens - formed by a symbiosis between an algae and a single fungus - is too simple, suggests a new study. The study's results reveal that a third species also contributes to this symbiotic relationship, one of the oldest- and best-known symbioses on Earth. For more than 140 years, scientists have thought that lichens arise from algae or cyanobacteria living among the filaments of a fungus. Recently, it has been suggested that another species may play a role in forming complete, functioning thalli, the leafy or shrubby features of lichens. Here, to further investigate, Toby Spribille et al. turned to two species of lichens, Bryoria fremontii and B. tortuosa, which produce the toxic substance vulpinic acid. The production of this acid causes B. tortuosa to have a yellowish color compared to the brown hue it inspires in B. fremontii, and scientists have not understood what causes this. Recent phylogenetic analyses have failed to detect any substantial genetic sequence differences between the two types of lichen - in either their fungal species or their photosynthesizing (algal) counterpart. Therefore Toby Spribille et al. took 15 samples of both species from sites in Montana and conducted mRNA sequencing. To their surprise, their data revealed 506 genetic signatures reflective of the presence of Cyphobasidium, a type of yeast belonging to the Basidiomycete phylum. Upon broader investigation, the team found other Basidiomycete lineages associated with 52 lichen genera from six continents. Analysis of Cyphobasidium by comparing it to its closest relatives suggests that it may have evolved around the time many lichens did, hinting at a long evolutionary history among it and the other species that make up lichens. When the researchers removed the Cyphobasidium from lichen samples, they discovered dead cells within the cortex of the lichen, suggesting that the yeast's presence is vital to the lichen's health. The yeast cells, it turns out, form the characteristic cortex of the lichen thallus and may be important for its shape.

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